This invention relates in general to electrostatography and, more specifically, to apparatus and process for detecting microdefects in xerographic photoreceptors with a contactless system.
Electrostatography is well known and includes, for example electrography and electrophotography. In electrography, an electrostatic latent image is formed on a nonelectrophotographic imaging member by various means such as styli, shaped electrodes, ion streams and the like. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
In the art of xerography, a xerographic plate or photoreceptor comprising a photoconductive insulating layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990, the entire disclosure thereof being incorporated herein by reference. A photosensitive member is described in this patent having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
The photoreceptors are usually multilayered and comprise a substrate, an optional conductive layer (if the substrate is not itself conductive), an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer and, in some belt embodiments, an anti-curl backing layer.
Although excellent toner images may be obtained with multilayered photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, reduced life would occasionally be encountered during extended cycling. Surprisingly, cycling of photoreceptors made up of identical materials but differing in overall size and use in different copiers, duplicators and printers exhibited different life spans where one of the causes of failure was dark decay. Moreover, photoreceptors from different production runs had different life spans when cycled to the point of dark decay failure in any given copier, duplicator and printer. Since photoreceptor properties can vary from one production run to another and also during cycling, copy quality in many machines is maintained by feedback control system which constantly adjusts the machine operating parameters to compensate for the variations in the dark decay electrical characteristic of any given photoreceptor. Thus, photoreceptor life is partially governed by the design of the control system and this leads to different life spans in different machines for the same photoreceptor where failure is due to unacceptable dark decay. However, even the control system of any given machine cannot compensate for variations in photoreceptor dark decay characteristics that extend outside the operating range of the control system.
In the production of electrophotographic imaging members the complex nature of the manufacturing process renders unpredictable electrical characteristics of the coated photoreceptor from batch to batch and from month to month. For example, reduction of photoreceptor life due to changes in environment affects the installation or adjustment of new coating applicators or the initial use of a newly prepared batch of coating material for one of the many layers of the photoreceptors such as the hole blocking layer, charge generating layer, or charge transport layer are difficult to identify within a reasonable length of time subsequent to the point in time that the photoreceptor comes off the production line.
In a photoreceptor, many types of microdefects can be a source of xerographic image degradation. These microdefects can be occlusions of particles, bubbles in the coating layers, microscopic areas in a photoreceptor without charge generator layer, coating thickness nonuniformities, dark decay nonuniformities, light sensitivity nonuniformities, and charge deficient spots (CDS's). This last type of defect, charge defficient spots, or CDS's are localized areas of discharge without activation by light. They can cause two types of image defects, depending on the development method utilized. Charge deficient spots usually can be detected only electrically or by xerographic development and so far have eluded microscopic or chemical detection.
In discharged area development, the photoreceptor is negatively charged. An electrostatic latent image, as a charge distribution, is formed on the photoreceptor by selectively discharging certain areas. Toner attracted to discharged areas develops this latent image. Laser printers usually work on this principle. When charge deficient spots are present on the photoreceptor, examination of the final image after toner transfer form the photoreceptor to a receiving member such as paper reveals dark spots on a white background due to the absence of negative charge in the charge deficient spots.
In charged area development, usually used in light lens xerography, the toner image is formed by developing the charged areas on a photoreceptor. After transfer of the toner image to a receiving member such as paper, the charge deficient spot on the photoreceptor will result in a small white spot in a black background called a microwhite, which is not as noticeable as a "microblack" spot, characteristic of discharged area development.
One technique for detecting charge deficient spots in photoreceptors from a specific production run is to actually cycle the photoreceptor in the specific type of copier, duplicator and printer machine for which the photoreceptor was fabricated. Generally, it has been found that actual machine testing provides the most accurate way of detecting charge deficient spots in a photoreceptor from a given batch. However, machine testing for detecting charge deficient spots is a very laborious and time consuming process which requires involving hand feeding of sheets by test personnel along with constant monitoring of the final quality of every sheet. Moreover, accuracy of the test results depends a great deal upon interpretations and behavior of the personnel that are feeding and evaluating the sheets. Further, since machine characteristics vary from machine to machine for any given model or type, reliability of the final test results for any given machine model must factor in any peculiar quirks of that specific machine versus the characteristics of other machines of the same model or type. Because of machine complexity and variations from machine to machine, the data from a test in a single machine is not sufficiently credible to justify the scrapping of an entire production batch of photoreceptor material. Thus, tests are normally conducted in three or more machines. Since a given photoreceptor may be used in different kinds of machines such as copiers, duplicator and printers under markedly different operating conditions, the charge deficient spots detection based on the machine tests of a representative test photoreceptor sample is specific to the actual machine in which photoreceptors from the tested batch will eventually be utilized. Thus, photoreceptor tests on one machine will not necessarily predict whether the appearance of charge deficient spots will occur if the same type of photoreceptor were used in another different type of machine. Thus, for a machine charge deficient spot test, the test would have to be conducted on each different type of machine. This becomes extremely expensive and time consuming. Moreover, because of the length of time required for machine testing, the inventory of stockpiled photoreceptors waiting approval based on life testing of machines can reach unacceptably high levels. For example, a batch may consist of many rolls, with each roll yielding thousands of belts. Still further delays are experienced subsequent to satisfactory charge deficient spot testing because the webs must thereafter be formed into belts, packaged and shipped.
Another test method utilizes a stylus scanner such as that described by Z. D. Popovic et al., "Characterization of microscopic Electrical Defects in Xerographic Photoreceptors", Journal of Imaging Technology, vol. 17, No. 2, April/May, 1991, pp. 71-75. The stylus scanner applies a bias voltage to a shielded probe, which is immersed in silicone oil and is in contact with the photoreceptor surface. The silicone oil prevents electrical arcing and breakdown. Current flowing through the probe contains information about defects, and scanning speeds up to 6.times.6 mm.sup.2 in about 15 minutes were achieved. Although the stylus scanner is a highly reproducible tool which enabled some important discoveries about the nature of charge deficient spots, it has the basic shortcoming of low speed.
Many attempts have also been made in the past to reduce the time of scan by designing contactless probes. For example, a probe has been described in the literature and used for readout of xeroradiographic (X-ray) amorphous selenium plates, e.g. W. Hillen, St. Rupp, U. Schieble, T. Zaengel, Proc. SPIE, Vol. 1090, Medical Imaging III, Image Formation, 296 (1989); W. Hillen, U. Schieble, T. Zaengel;. Proc. SPIE, Vol. 914, Medical Imaging II, 253 (1988); U. Schieble, W. Hillen, T. Zaengel;. Proc. SPIE, Vol. 914, Medical Imaging II, 253 (1988); U. Schieble, T. Zaemge;. Proc. SPIE, Vol. 626, Medicine XIV/PACS IV, 86 (1986); These probes rely on reducing the distance of a probe to a photoreceptor surface in order to increase resolution of the measurements. The typical distance of the probe to the photoreceptor surface is 50-150 micrometers. In order to avoid air breakdown the ground plane of a xeroradiographic plate is biased appropriately to provide approximately zero voltage difference between the probe and photoreceptor surface.
Thus, there is a need for higher scanning speeds without arcing for applications such as electrostatographic member production monitoring.